This invention relates to processes for the reversible hydrogenation of pi-conjugated substrates to provide for the storage and release of hydrogen at practical operating temperatures and pressures, particularly for supplying hydrogen to fuel cells. Hydrogen is a widely used chemical commodity in the chemical and petroleum processing industries, but with the relatively recent development of fuel cells it is increasingly also being considered as a viable “clean” energy source. Stationary fuel cells can be supplied with hydrogen from on-site natural gas reformers or via existing hydrogen pipeline sources. However, for mobile vehicular systems, a practical and effective method for storing hydrogen to power an on-board fuel cell is required. The transport of hydrogen as a cryogenic liquid, although technologically well established, is an energy-intensive process which results in a significantly higher cost of the delivered gas. Hydrogen is also conventionally transported as a compressed gas in steel cylinders, but the storage capacity is relatively low. Higher gravimetric storage amounts, but at relatively low volumetric densities, can now be achieved with hydrogen gas at very high pressures up to 10,000 psi (690 bar) in light-weight containers made of very high strength composite materials. There are considerable energy costs in thus compressing the gas as well as potential issues regarding consumers' acceptance of systems that contain hydrogen at such elevated pressures.
It is thus necessary and highly desirable to devise a means of storing hydrogen safely, at an adequate gravimetric and volumetric storage density and with a minimal consumption of energy. This may be accomplished by “containing” the hydrogen in a suitable solid, or potentially even in liquid sorbent media, compositions which have a substantial but reversible affinity for the gas. The gas is contacted with the sorbent at modest temperatures and hydrogen pressures and is released for use as required, by lowering the system's hydrogen partial pressure at the same or a higher temperature. Hydrogen uptake by the sorbent is usually an exothermic process, while the release of hydrogen for use requires at least the corresponding input of thermal energy which can be met from the fuel cell's waste heat. Thus, in contrast to compressed hydrogen-based storage systems, the necessary energy needed for containing the hydrogen (i.e. its heat of adsorption) can thus largely be met without significant consumption of higher grade electrical energy for compression. Also, there is a considerably increased safety factor in a sorbed hydrogen. Since any desorption process is endothermic, it will be naturally self-retarding and the hydrogen will not spontaneously totally desorb without an external input of heat.